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# Quantum teleportation and relativity

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I have been thinking about a video I saw on quantum mechanics and this is stumping me. Suppose a quantum teleportation device has been invented allowing instantaneous transporting of objects. Imagine a spaceship has been travelling away from earth at a pct of c so that an hour has passed(ship time) while 2 hours have passed(earth time). Let's say the ship q-teleports a message to earth saying their whatsit is broken and they need a new one. The message travels instantly so it arrive at 1 hr after ship left(earth time) which would be 30 min(ship time).Earth instantly sends the new one so it arrive 30 min before the old one broke(ship time).The ship assumes earth just sent them a spare and so 30 min later when the old one breaks they replace it with the new one and no message is ever sent. Does the universe implode or what?

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First you say 1 hour has passed ship-time, 2 hours earth-time, then you say the ship's message was sent 30 min ship-time, 1 hour earth-time. ???

Did the part break/message sent 1 hour ship-time or 30 min ship-time?

Edited by StarMountainKid

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From a non-linear, non-subjective point of view, when the first message is sent, it is simultaneously both an hour of time for those on the ship, and two hours for those on Earth. Thus when the replacement is sent, it arrives just after an hour for those on the ship and just after two hours on Earth.

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First you say 1 hour has passed ship-time, 2 hours earth-time, then you say the ship's message was sent 30 min ship-time, 1 hour earth-time. ???

Did the part break/message sent 1 hour ship-time or 30 min ship-time?

Well I'm not sure I'm saying it right. The ship after an hour of elapsed time sent an instant message to earth. If earth receives it at an hour of elapsed time it would have effectively went backwards in time relative to the ship, unless I'm seeing it wrong

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From a non-linear, non-subjective point of view, when the first message is sent, it is simultaneously both an hour of time for those on the ship, and two hours for those on Earth. Thus when the replacement is sent, it arrives just after an hour for those on the ship and just after two hours on Earth.

I was thinking that might be the case but I'm still not sure. Clearly the ship's time would be going slower relative to the earth's. An instant message from the ship to earth is what is messing with my head. Would the one elapsed hour not still hold in the case of an instant message to earth? No matter what is happening on earth only an hour has passed when the message is sent. Maybe I'm overthinking this

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In my view, Insanity's answer is correct. This is the conclusion I reached before I started nit-picking your OP, sorry. The two clocks in this case are synchronized in the sense that one clock is ticking twice as fast as the other clock. If it's 12:00 ship time when the message is sent, it is received on earth at 2:00. Instantly the part is sent at 2:00 earth time and received at12:00 ship time.

I'm trying to think of some way a time paradox could occur in this scenario.

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In my view, Insanity's answer is correct. This is the conclusion I reached before I started nit-picking your OP, sorry. The two clocks in this case are synchronized in the sense that one clock is ticking twice as fast as the other clock. If it's 12:00 ship time when the message is sent, it is received on earth at 2:00. Instantly the part is sent at 2:00 earth time and received at12:00 ship time.

I'm trying to think of some way a time paradox could occur in this scenario.

Yeah, I think you both have the right of it. I guess I don't get a Nobel for inventing a time machine this year. Still from earth an observer would say the ship's time would be going slower. On the ship however they would feel motionless and it would appear to them the earth was rushing away at whatever velocity so to them would it not appear earth's time was going slower? Obviously both can't be true though relativity says both are equally valid. Sorry if I appear dense but even after all these years of thinking about this I still confuse myself sometimes

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Yeah, I think you both have the right of it. I guess I don't get a Nobel for inventing a time machine this year. Still from earth an observer would say the ship's time would be going slower. On the ship however they would feel motionless and it would appear to them the earth was rushing away at whatever velocity so to them would it not appear earth's time was going slower? Obviously both can't be true though relativity says both are equally valid. Sorry if I appear dense but even after all these years of thinking about this I still confuse myself sometimes

Thats a really good question...though I cant answer it . No doubt someone will.

Your OP has me asking this ... Is there such a thing as instantaneous light speed? Or to put it another way does light experience acceleration?

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Thats a really good question...though I cant answer it . No doubt someone will.

Your OP has me asking this ... Is there such a thing as instantaneous light speed? Or to put it another way does light experience acceleration?

Well from the above you can probably tell I'm no physicist, but I have read voraciously on the subject over the years and never have I heard of a photon doing anything but moving at light speed so I would say when a photon is emitted from whatever source it is instantly travelling at light speed
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Still from earth an observer would say the ship's time would be going slower. On the ship however they would feel motionless and it would appear to them the earth was rushing away at whatever velocity so to them would it not appear earth's time was going slower? Obviously both can't be true though relativity says both are equally valid.

I think the answer to this question is that the space ship had to accelerate to its speed relative to the earth. The earth did not accelerate away from the ship.

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Well from the above you can probably tell I'm no physicist, but I have read voraciously on the subject over the years and never have I heard of a photon doing anything but moving at light speed so I would say when a photon is emitted from whatever source it is instantly travelling at light speed

Which is what we see from lasers, though sometimes it is hard to get your head around the concept of going from a state of nonexistance to light speed in zero time. There might be some clue to the possibility of time travel here, but in these things I am Ralph Wiggum and my head hurts

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Which is what we see from lasers, though sometimes it is hard to get your head around the concept of going from a state of nonexistance to light speed in zero time. There might be some clue to the possibility of time travel here, but in these things I am Ralph Wiggum and my head hurts

Yeah it kinda makes some veins pop in my head too
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Yeah, I think you both have the right of it. I guess I don't get a Nobel for inventing a time machine this year. Still from earth an observer would say the ship's time would be going slower. On the ship however they would feel motionless and it would appear to them the earth was rushing away at whatever velocity so to them would it not appear earth's time was going slower? Obviously both can't be true though relativity says both are equally valid. Sorry if I appear dense but even after all these years of thinking about this I still confuse myself sometimes

Crudely speaking they are both true (technically the Earth isn't in an inertial frame, but if we can replace the Earth with another space ship moving at a constant velocity in the other direction than the comparison is correct). There is can only be a maximum of 1 point of contact between two objects travelling in different directions, so there is no paradox.

Your OP has me asking this ... Is there such a thing as instantaneous light speed? Or to put it another way does light experience acceleration?

Light can experience acceleration but only transverse to the direction of travel, i.e. the light path will curve. (Changing direction is a form of acceleration.)

When thinking about light begin ``created'' and instantly travelling at light speed it might help to visualize ripples on a pond. When you throw a rock into a pond the ripples are ``created'' and instantly travel at the appropriate wave propagation speed for the water/air boundary. Of course we can see that the ripples aren't discrete objects in and of themselves, rather they are a propagating disturbance in the surface of the water caused by the initial rock.

Photons can likewise be thought of as disturbances in the electromagnetic field, caused by shaking up charged particles.

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Crudely speaking they are both true (technically the Earth isn't in an inertial frame, but if we can replace the Earth with another space ship moving at a constant velocity in the other direction than the comparison is correct). There is can only be a maximum of 1 point of contact between two objects travelling in different directions, so there is no paradox.

Light can experience acceleration but only transverse to the direction of travel, i.e. the light path will curve. (Changing direction is a form of acceleration.)

When thinking about light begin ``created'' and instantly travelling at light speed it might help to visualize ripples on a pond. When you throw a rock into a pond the ripples are ``created'' and instantly travel at the appropriate wave propagation speed for the water/air boundary. Of course we can see that the ripples aren't discrete objects in and of themselves, rather they are a propagating disturbance in the surface of the water caused by the initial rock.

Photons can likewise be thought of as disturbances in the electromagnetic field, caused by shaking up charged particles.

Which analogy I used as a highly theoretical way of suggesting FTL communications in the thread about SETI. But in your analogy we have photons having no mass?, if so, then were does that put the idea of "light sails" if photons have no force behind them, becalmed?

Edited by Kaa-Tzik

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As a photon has energy it has momentum. A photon's energy is defined by its wavelength.

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[/media] Edited by StarMountainKid

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As a photon has energy it has momentum. A photon's energy is defined by its wavelength.

Thanks, that's now clear, I just had a moment of doubt

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Crudely speaking they are both true (technically the Earth isn't in an inertial frame, but if we can replace the Earth with another space ship moving at a constant velocity in the other direction than the comparison is correct). There is can only be a maximum of 1 point of contact between two objects travelling in different directions, so there is no paradox.

Light can experience acceleration but only transverse to the direction of travel, i.e. the light path will curve. (Changing direction is a form of acceleration.)

When thinking about light begin ``created'' and instantly travelling at light speed it might help to visualize ripples on a pond. When you throw a rock into a pond the ripples are ``created'' and instantly travel at the appropriate wave propagation speed for the water/air boundary. Of course we can see that the ripples aren't discrete objects in and of themselves, rather they are a propagating disturbance in the surface of the water caused by the initial rock.

Photons can likewise be thought of as disturbances in the electromagnetic field, caused by shaking up charged particles.

A couple of points : First about the single point of contact, is this not changed by the hypothetical quantum teleporter connecting the two? And second about light bending, is it not more correct to say the space the light travels through is actually what bends while the light merely follows a straight line through the bent space?

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As a photon has energy it has momentum. A photon's energy is defined by its wavelength.

[media=]

[/media]

To say a photon has momentum, would this be the same as saying it has inertia?

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A couple of points : First about the single point of contact, is this not changed by the hypothetical quantum teleporter connecting the two?

Not really.

However the single point of contact is justification for faster-than-light communications to be impossible.

If you can have faster-than-light communication it is possible to send messages back in time (for the appropriate arrangement of message relays travelling at various velocities relative to each other).

And second about light bending, is it not more correct to say the space the light travels through is actually what bends while the light merely follows a straight line through the bent space?

It might be, I guess. The two are more or less equivalent.

To say a photon has momentum, would this be the same as saying it has inertia?

Yes. A photon has no rest mass. It does have a relativistic mass. It therefore has no inertia when at rest (and consequently takes 0 energy to accelerate it to a finite speed, which I suppose is one argument why photons always travel at light speed).

Once it is moving at light speed it has a finite mass that increases with photon energy (decreases with increasing wavelength).

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Thanks sepulchrave that was extremely interesting. This stuff fascinates me though I struggle with comprehending it sometimes

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Just take in mind that the name "quantum teleportation" is a little bit misleading - neither it is happening at superluminal speed, nor any object is "cloned".

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If you are talking about quantum entanglement which is probably how a quantum teleporter would have to work by I thought the changes were basically instantaneous and FTL

http://en.wikipedia.org/wiki/Quantum_entanglement

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Exactly. I realize nothing is cloned but rebuilt by particles exactly like the ones being sent and therefore indistinguishable from the original. Correct? And entanglement is instantaneous though nothing actually travels ftl. At least if I'm understanding this right. The point is moot though because I was hoping to combine Q-teleportation with special relativity to get a time machine but apparently that aint gonna happen

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Quantum teleportation and quantum entaglement are two completely different processes:

Teleportation is when one system sends info about its internal state to othet system - this happens with speed of light.

Entaglement is when we measure simultaniously two parts of a system and the two measurements are interdependent - the result of one depends on the result of the other. This dependence happens instantaneously and FTL, but!!!!! It can't be used to send information.

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The speed of light, though on earth seems extremely fast, is very slow with respect to the universe.

Gaze outside into a dark night. The stars you see are not in their viewed position, at all. Far from it. Some may well be on the other side of the earth. Just the way it is with the SLOW speed of light.

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